Copper is in wide use, ranging from decorative and ornamental applications to its applications in the electronic industry, and like most other metals it undergoes oxidation when exposed to the air. This causes a decline in its solderability and electrical conductivity, which has prompted the implementation of various countermeasures.
For example, in the electronics industry this deterioration in solderability and electrical conductivity of the copper foil used for printed circuit boards has been prevented by inhibiting oxidation of the copper surface by the application thereto of an organic inhibitor as a surface treatment.
However, the recent direction in the electronics industry has been toward lighter, thinner, smaller, and higher-performing components, and this has required printed circuit boards to support higher densities and smaller conductor paths. This in turn has increased the requirements in the areas of solder wettability, solder resistance, durability of adhesion to the copper foil, and migration resistance of the copper. These issues are usually handled by inducing an improvement in these properties by directly treating the surface of the copper foil with an antioxidant or copper inhibitor or by adding an antioxidant or copper inhibitor to the adhesive.
For example, Japanese Patent Application Laid Open Kokai or Unexamined! Number Hei 1-251785 251,785/1989! discloses a surface treatment method that uses an organic inhibitor. In this method, copper migration is inhibited by dipping the printed-circuit board, while heating under elevated pressure, in a solution that contains a metal deactivator or copper inhibitor. However, despite infiltration of the treatment agent while heating under elevated pressure, this method has difficulty achieving a uniform infiltration and as a result produces a variable migration resistance. In addition, even though the treatment agent infiltrates into the adhesion interface, etc., due to the application of heat and pressure, the durability of adhesion to the copper under high temperature/high humidity conditions is unsatisfactory.
Japanese Patent Application Laid Open Kokai or Unexamined! Number Hei 5-65466 65,466/1993! discloses a method that inhibits copper migration and improves antitracking through the addition of a triazinethiol copper inhibitor to the adhesive for the copper-clad laminate. While this type of adhesive does prevent copper migration in those regions where it directly contacts the copper, almost no effect is obtained in regions not in contact with the adhesive.
Thus, no surface treatment agent is currently available that is able to provide the surface of copper-containing metals with a satisfactory performance from all perspectives, i.e., oxidation resistance, migration resistance, solder resistance, and robustness of adherence.